A Clean Intersection

The plumb thread hung steady against the curb joint, casting a hairline shadow across the sand. Midday glare threw the crimped edge of the bottle cap into sharp focus. I grounded the barrel of my long rifle into the silt, leaning forward to observe the crossing. The plumb shadow and the cap gnomon intersected at a single point, two finger-widths out from the stone. Using the tip of a stiff broom bristle from my pack, I etched a fine alignment mark straight into the packed grit beneath the crossing. It held without crumbling. The baseline is fixed.